This has been explained in the mailing lists. The switches are configured by OpenWrt using 802.1q VLANs, so you get two interfaces:

eth0.1 => LAN

eth0.2 => WAN

Now qMp uses 802.1ad (i.e. QinQ) VLANs, so that you can apply them on top of the 802.1q VLANs used for managing the switches. Therefore, if you want to use the [eth0.2 interface/WAN port] for meshing, you just configure it on the web interface as if it was a regular (i.e. non-switched) interface (e.g. eth2).

For example, a TP-Link TL-WDR3600 (and surely with 4300) with a testing image, doesn't connect to the Mesh via cable.

Indeed, it does. Please have in mind that we are using 802.1ad VLANs for tagging the BMX6 packets, so the testing firmware binaries are not backwards-compatible with previous qMp versions.

Previously the last port (4) of the switch was used for Mesh via cable, configured with vlan 12.What is the way to do it now without hooks nor setting it manually?

The VLAN12 hook was a dirty workaround which does not make sense anymore. You can mesh via any of the ports, you just have to configure it on the web interface (or in /etc/config/qmp).